1. Field
This invention relates generally to an improved polyolefin microporous breathable film and method of making same. More specifically, this invention is directed toward a process by which increased Water Vapor Transmission Rate (WVTR) and enhanced film appearance can be realized with substantially the same film formulation and orientation.
2. Background
Preparation of films having good WVTR from highly filled polymers, usually polyolefins, is well known in the art. In the past, a combination of polyolefin, usually a polyethylene, with a filler, usually CaCO.sub.3, is widely used as a film with good WVTR, often, but not necessarily, in combination with non-woven polymers for use in diapers, adult incontinence devices, feminine hygiene articles, surgical garments, housewrap composites, protective apparel, roofing materials and the like.
The use of interdigitating rolls to orient films or non-wovens is also well known in the art. In some cases this process is referred to as cold stretching. To increase the WVTR of films, while employing interdigitating technology, it has been necessary to increase the level of filler in the polyolefin/filler blend, or to increase the depth of interengagement of the orienting rollers--both of which have technical limits, and which may have a serious negative impact on important physical properties of the resulting film. The technical limits of depth of engagement of the interdigitating rolls and CaCO.sub.3 loading restrict film breathability level.
Also, it is desirable for many applications of breathable film, such as disposable diapers, adult incontinence products, and feminine hygiene devices, that some visual evidence of a difference between breathable and non-breathable films exist. It is thought that this product differentiation could be of benefit to the consumer, as well as the manufacturer of the disposable products.